The Big Idea
In the absence of a net force, objects move in a straight line. If they turn — that is, if their velocity changes, even only in direction — there must be an applied force. Forces which cause objects to turn around continuously in a circle are known as centripetal forces. When an object moves in a circle its velocity at any particular instant points in a direction tangent to the circle. The acceleration points towards the center of the circle, and so does the force acting on it. This is only natural, when you think about it — if you feel a force pushing you towards your left as you walk forward, you will walk in a circle, always turning left.
The reason the Earth orbits the sun is due to the Universal Law of Gravity and centripetal motion. The Universal Law of Gravity was a great achievement in the history of mankind. It unified the 'heavenly bodies' and the 'Earthly bodies'. Isaac Newton showed that an apple falling to the ground and the moon orbiting Earth are both explained by the same laws of physics.